The present invention generally relates to controlling head movement and, more particularly, to controlling head movement within a servo control loop.
Hard disk drives contain a plurality of magnetic heads that are coupled to surfaces of rotating disks. The heads can magnetize and sense the magnetic fields of the disk surfaces to write and read data, respectively. The heads each have an air bearing surface that cooperates with a flow of air generated by the rotating disks to create an air bearing to avoid contact with the adjacent disk surfaces. The air bearing therefore reduces mechanical wear between the heads and the disk surfaces.
The data is typically stored on annular tracks that extend radially across each disk surface. To access data in different tracks, the drive moves the heads across the disk surfaces in accordance with a seek routine.
The surfaces of the disks may have protrusions caused by, for example, disk defects or contaminants embedded into or residing on the disk surface. Impact between the heads and the protrusion may create heat that degrades the read back signal. This phenomenon is referred to as a thermal asperity. Additionally, such impact may create an instability in the air bearing and cause erroneous reading or writing of data on the affected disk surface. For these or other reasons, some disk drives are configured to avoid reading or writing data on tracks that are known to contain thermal asperities.